Are there any instances that disable downvoting?

Yes, I think Beehaw for example has no downvotes (or doesn't show them).

Beehaw and my instance are the only ones that I know of

https://lemmy.one/

This page, "Awesome Lemmy instances", has a big list of instances with some info, including whether they allow downvotes. I'm not sure if it's up to date though.